Continuous glass fibers are formed by attenuating molten glass through apertures or tips at the bottom of a heated, metal container known as a "bushing". The bushing typically is surrounded by refractory materials to provide thermal and electrical insulation as well as mechanical support for the bushing.
The bushing assembly must withstand the corrosive operating environment provided by the molten glass contained therein and elevated operating temperatures without degradation of its electrical or mechanical components. These stringent requirements typically are met by fabricating the glass contacting portions of the bushing assembly from precious metals such as platinum or platinum alloys. However, the operating environment somewhat affects even these materials. Oxidation loss, volatilization, and migration of the precious metal into surrounding refractory materials as well as sagging or creep (high temperature deformation) of the bushing decrease bushing performance as well as shorten the useful life of the bushing assembly. Attempts to address such problems are discussed below.
U.S. Pat. No. 4,382,811 discloses a method of coating metal parts to be used in contact with molten glass with a composition consisting essentially of at least 60% by weight of chromium oxide, from 0 to 40% by weight of a nickel-based alloy and from 0 to 20% by weight of metal oxides (col. 1, lines 34-39). This patent does not discuss glass fiber forming apparatus.
European Patent Publication No. 0381179 discloses electrophoretic deposition of a ceramic coating on the non-glass contacting surfaces of a bushing for glass fiber production (page 2, col. 1, lines 24-28).
U.S. Pat. Nos. 3,685,978 and 3,859,070 disclose a bushing having a refractory coating on the exterior surfaces of the bushing, including the bottom surface of the bushing and the sidewalls, to prevent oxidation losses of the precious metal and reduce the tendency of the bushing to sag during use (col. 5, lines 17-27 and col. 6, lines 16-20 of the '978 patent; and col. 6, lines 58-69 and col. 8, lines 11-15 of the '070 patent). The coated bushing also has a high temperature insulating, fibrous material disposed about and in a touching relationship with the coating on the bushing except at the tip plate (col. 6, lines 20-26; col. 8, lines 15-24, respectively). The fibrous material is "used to insulate and allow for expansion of the bushing during service . . . " (col. 6, lines 23-24; col. 8, lines 19-20, respectively). The inside of the bushing also may be coated with a refractory material resistant to glass attack to strengthen the structure and prevent oxidation losses (col. 6, lines 58-63; col. 8, lines 42-46, respectively). Neither of these references discloses a bushing having a flange.
U.S. Pat. No. 4,846,865 discloses a bushing having a ceramic coating such as yttrium oxide-stabilized zirconium oxide "applied on the outwardly disposed surfaces, or portions of the bushing, that is the nonglass contacting portions of the bushing . . . " (col. 3, lines 23-26). The bushing is heated to a temperature of about 2732.degree. F. for about six hours to further sinter the coating to enhance bonding of the coating to the precious metal (col. 4, lines 45-53).
Degradation of the refractory material surrounding the bushing, contributing to localized electrical resistance changes in the bushing and non-uniform heating of the molten glass within it, is a particularly troublesome problem affecting bushing longevity and productivity that is inadequately addressed by the foregoing references.
In a direct-melt operation, the flanged bushing can be positioned directly below a metal-lined bore in an insulating block or "bushing block". Electrical isolation is necessary to prevent electrical contact between the bushing and the metal liner. Without electrical isolation, the electric current which heats the metal bushing would be diverted to heat the metal liner causing (1) disruption of temperature control of the bushing tip plate and (2) localized over-heating of the bushing assembly between the flange and the liner. Disruption of temperature control of the bushing tip plate can cause the temperature of the glass adjacent the tip plate to become non-uniform which contributes to fiber break-out during the attenuation process. As used herein, "break-out" means that one or more continuous fibers break during the attenuation process. Localized over-heating of the bushing due to an increase in resistive heating in the area of contact between the flange and the bushing block liner can cause glass leakage from between the flange and the liner. When glass leakage occurs, the attenuation process must be discontinued and the bushing replaced. Both break-outs and glass leakage result in lost production time, decreased production efficiency, and increased materials and maintenance costs.
To electrically isolate the bushing block liner from the flange, a low bulk density, fibrous, electrically insulating gasket material, such as FIBERFRAX 970 J insulating material.sup.1, typically has been interposed between the flange and the liner. As used herein, "low bulk density" means insulating materials having a bulk density of less than about 1.0 gram per cubic centimeter (g/cc) and "high bulk density" means insulating materials having a bulk density greater than about 2.0 g/cc. FNT .sup.1 FIBERFRAX 970 J is an aluminosilicate fiber-containing insulating material which is commercially available from The Carborundum Company of Niagara Falls, N.Y.
Low bulk density, fibrous electrically insulating materials are not desirable as gaskets in fiber forming operations because of their poor structural integrity. As used herein, "poor structural integrity" means that the insulating material is friable, easily crumbled or pulverized and "good structural integrity" means that the insulating material resists crumbling or pulverization. See Webster's New Collegiate Dictionary (1977) at page 460, which is hereby incorporated by reference.
During installation of the bushing beneath the bushing block, irregularities on the surface of the bushing block liner can penetrate the low bulk density, fibrous, insulating gasket material and contact the bushing flange. During the fiber forming operation, contact between the metallic bushing block liner and bushing flange can disrupt the electric current directed through the bushing and thereby disrupt temperature control of the bushing tip plate resulting in non-uniform heating of the molten glass and increased break-outs. Typically, thick or multiple layers of low bulk density, fibrous insulating material have been interposed as a gasket between the bushing block liner and the bushing flange to prevent contact therebetween. As used herein, a "thick layer" of insulating gasket material has an average thickness greater than about 1.0 mm (0.040 inches) and a "thin layer" of insulating gasket material has an average thickness of less than about 0.5 mm (0.020 inches).
Also, a thick layer of low bulk density, fibrous, insulating gasket material between the bushing block liner and the bushing flange can create a void space or gap in which molten glass can accumulate. Chemical reactions occurring between the molten glass accumulated in the gap and the surrounding insulating gasket material can result in devitrification or crystallization of the molten glass into solid particles. Such particles, which are not bonded to the bushing block liner or the flange, can be eroded by and entrained in the adjacent flowing molten glass and contribute to break-out of the fibers. Also, the flowing molten glass can erode the low bulk density, fibrous, insulating material itself. Thick layers of the low bulk density, fibrous, insulating material increase the amount of surface area subjected to the erosive action of the molten glass.
Since poor production efficiency and production downtime due to fiber break-out and replacement of failed bushings results in costly waste of labor and materials, an electrically insulating material which has good structural integrity and resistance to erosion and chemical interaction with molten glass is needed for use in a bushing assembly as a gasket between the bushing flange and the metal liner on the bushing block.